Try everything else listed above, but if you can't get it to stop, this is what I heard just this week-end from a competent designer/builder:
Naim tells their dealers to only demo with one unit plugged in, as this cross-talk is inherent in their grounding/design method.
Now, for you Naim folks, I like the sound, but if this is a problem, the unit is probably NOT defective. If my guy is right, then your unit is just working the way that Naim components work and is okay. I hope one of the other suggestions is helpful, but if not, don't freak out, it';s just the way it is: anything else turned on and playing is going to bleed through at a low level.
Just what I heard. Please don't crucify me for passing it on.